Seat belt tension sensor

ABSTRACT

A seat belt tension sensor for measuring the amount of tension in a vehicle seat belt. The seat belt tension sensor has a housing and an anchor plate that is mounted in the housing. The housing moves relative to the anchor plate. A magnet is held by the anchor plate. A sensor is mounted to the housing. The sensor generates an electrical signal in response to relative movement between the housing and the anchor plate. The electrical signal changes in proportion to the amount of tension on the seat belt. A spring is mounted in association with the housing and the anchor plate. The spring biases the housing from the anchor plate. An integral connector extends from the housing.

CROSS-REFERENCE TO CO-PENDING AND RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/388,816, filed Mar. 14, 2003 and titled, “Seat Belt TensionSensor Having Shock Isolation”, which is a continuation-in-part of U.S.patent application Ser. No. 09/923,151, filed Aug. 6, 2001, now U.S.Pat. No. 6,578,432 and titled, “Seat Belt Tension Sensor”, which is acontinuation-in-part of U.S. patent application Ser. No. 09/884,615,filed Jun. 19, 2001 and titled, “Seat Belt Tension Sensor”, now U.S.Pat. No. 6,647,811.

This application is related to U.S. Pat. No. 6,729,194, issued May 4,2004 and titled, “Hall Effect Seat Belt Tension Sensor”.

The foregoing pending applications and issued patents are hereinincorporated by reference in their entirety.

BACKGROUND

1. Field of the Invention

This invention relates to an automobile sensor for detecting themagnitude of a tensile force in a seat belt used in a car seat, and inparticular to a sensor that can detect the magnitude of tension in aseat belt and provide an electrical signal that is representative of themagnitude of tensile force.

2. Description of the Related Art

Air bags have been heralded for their ability to reduce injuries andsave lives. However, since their incorporation into automobiles, aproblem has existed with people of smaller size and small children. Airbags are designed to cushion the impact of occupants and thus reduce theinjuries suffered. However, the force needed to properly cushion theoccupant varies based on the size and position of the person.

For example, a larger person requires the bag to inflate faster and thuswith more force. A smaller person may be injured by a bag inflating atthis higher inflation force. A smaller person is more likely to besitting close to the dashboard and would therefore stand a higher chanceof being injured by the impact of the inflating bag, as opposed to thepassenger hitting the fully inflated bag to absorb the impact of theaccident. An average-sized person can also be injured by an airbaginflation if they are leaning forward, as for example, if they areadjusting the radio.

Because of the concern over injury to passengers in these situations,the National Highway Transportation Safety Administration (or NHTSA), anadministrative agency of the United States, is instituting rules underFMVSS 208 requiring the air bag deployment system to identify thepassenger size and position and inflate the air bag accordingly.

One way to accomplish this task is to use a seat belt tension sensor inconjunction with an occupant weight sensor. The weight sensor canprovide an indication of the force placed by an occupant on the seat.However, if the seat belt is unduly tightened, it can place anadditional downward force on the passenger, creating an erroneous weightreading. Similarly, it is common for infant car seats to be securedtightly to the seat. In this circumstance, it is critical for the systemto recognize that the passenger does not warrant inflation of the airbag. By sensing the tension on the seat belt in addition to the weightreading from the seat, the actual weight of the occupant can bedetermined. This allows for the system to safely deploy the air bag.

SUMMARY

It is a feature of the present invention to provide a seat belt tensionsensor for use with a seat belt in a vehicle.

Another feature of the invention is to provide a seat belt tensionsensor that includes a housing with an anchor plate and a sensor mountedin the housing. The anchor plate holds a magnet and a sensor is mountedto the housing. The sensor generates an electrical signal in response torelative movement between the housing and the anchor plate. Theelectrical signal changes in proportion to the amount of tension on theseat belt. A spring biases the housing from the anchor plate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective exploded view of a seat belt tension sensor inaccordance with the present invention.

FIG. 2 is an assembled view of FIG. 1.

FIG. 3 is a perspective view of FIG. 2 with a cover removed.

FIG. 4 is a top view of FIG. 3.

FIG. 5 is an enlarged view of a magnet carrier of the present invention.

It is noted that the drawings of the invention are not to scale.

DETAILED DESCRIPTION

The present invention is a seat belt tension sensor. Referring to FIGS.1-4, a seat belt tension sensor assembly 20 is shown. Assembly 20 has ahousing 40 and anchor plate 60. Housing 40 is fastened between a seatbelt webbing 30 and a structural part of the vehicle such as a floor(not shown). The belt webbing 30 has an end 31, an end 32, a belt loop33 and stitching 34 that retains end 32.

Housing 40 has a bottom portion 41, top portion 42, flange 43, hole 44,spring channel 45, bearing rail 46 and sensor mounting area 47. A cavity48 is located within housing 40. The bottom portion 41 and top portion42 connect together to form housing 40 and are held together by snapfitting tabs 42 a in to slots 42 b. Alternatively, ultrasonic weldingalong lip 41A can connect portions 41 and 42 together. Housing portion42 has a recess or narrow portion 49 on an end of the housing where theseat belt wraps around.

An integral connector 50 extends from housing bottom portion 41.Connector 50 is integrally molded into housing portion 41 duringinjection molding of the housing. Integral connector 50 has a shroud 51that has a recess 52. Shroud 51 has shroud ends 51A and 51B. Severallatch tabs 54 are mounted on shroud 51. Plate 55 supports shroud 51.Shroud end 51A extends from one side of plate 55 and shroud end 51Bextends from the other side of plate 55. A molded support or bracket 53reinforces the attachment between integral connector 50 and housingportion 41. Molded support or bracket 53 extends between housing side41C and a plate 55. Plate 55 connects between shroud 51 and side 41C.Bracket 53 has ribs or webbing 53A that adds additional mechanicalstrength to bracket 53. Shroud 51 is spaced from side 41C by an airgap180 (FIG. 4).

Three electrically conductive metal terminals 56 have ends 56A and 56B.Terminals 56 are integrally molded into housing portion 41 duringinjection molding of the housing. Terminals 56 extend between sensormounting area 47 in housing 40 and recess 52. Terminal ends 56A extendinto sensor mounting area 47 and terminal ends 56B extend into recess 52of shroud 51. Housing 41, bracket 53, plate 50 and shroud 51 can bemolded from plastic. The plastic surrounds and support each terminal andinsulates the terminals. Terminal end 56A is adapted to be connected toan external wire harness 250 (FIG. 4). The wire harness would fit overshroud 51 and be retained by latch tabs 54. Wire harness 250 wouldconnect with an airbag controller (not shown).

In an alternative embodiment, terminals 56 could be press-fit intoshroud 51, support 53 and housing 41. In this example, terminals 56 areretained by friction between the terminals and the surrounding structureprimarily support 53.

The use of integral connector 50 has many advantages. Integral connector50 eliminates the need for a separate wire harness and connector. Sinceintegral connector 50 is rigidly held by bracket 53, a separate strainrelief mechanism is not required. If a wire harness and connector wasused, a separate strain relief mechanism would be required to preventthe wire harness from being pulled out of housing 40.

Integral connector 50 eliminates the need for a separate printed circuitboard because hall effect device 82 is mounted directly to terminals 56.Alternatively, a printed circuit board could be mounted by solderingdirectly to terminals 56 and the hall effect device mounted to theprinted circuit board.

A metal anchor plate 60 is fitted within housing 40. Anchor plate 60 hasa top surface 60A and bottom surface 60B. Anchor plate 60 includes ends61 and 62, a cutout 63, apertures 64 and 65 and sides 66 and 67. Anchorplate 60 further has edges 65A and 65B that are next to aperture 65. Arm68 extends between sides 66 and 67 and separates aperture 65 and cutout63. A projection 69 extends from arm 68 into cutout 63. Projection 71extends into cutout 63. Projection 71 and arm 68 define a sensormounting opening 72. Notch 73 is defined between projection 71 and anedge 74 of cutout 63.

Anchor plate 60 is mounted in cavity 48. Aperture 65 goes over andsurrounds flange 43. A gap 80 (see FIG. 4) is formed between flange 43and aperture 65.

Seat belt webbing 30 is attached through hole 44 and aperture 65. Theend 32 of webbing 30 is routed through hole 44 and aperture 65, wrappedback onto itself forming loop 33 and sewn with stitching 34 to securethe seat belt webbing to assembly 20.

A coil spring 90 is mounted in spring channel 45. Spring 90 has ends 91and 92. Spring channel 45 is defined by walls 94, 95 and 96 in housing40. Spring end 92 is mounted over projection 69. The other spring end 91rests against wall 96. Spring 90 is adapted to bias anchor plate 60 fromhousing 40 such that gap 80 is open. A slot 97 is located in wall 96.

A magnetic field sensor or hall effect device 82 is mounted to terminalend 56A and extends upwardly into sensor mounting area 47. Additionalelectronic components (not shown), such as resistors, capacitors,inductors, transistors or integrated circuits can also be attached toterminal ends 56A to condition the signal from the hall effect device82. Since, terminals 56 are insert molded, hall effect device 82 isretained in the proper position in sensor mounting area 47. Hall effectdevice 82 can be mounted to terminal ends 56A by soldering or laserwelding.

Hall effect device 82 and ends 56a can be encapsulated with a sealant 84such as silicone for protection.

A magnetic field generator or magnet assembly 100 includes a magnetcarrier 102 and a magnet 110. Magnet carrier 102 has a cavity 104 and aslot or mortise 106. The magnet carrier is preferably formed from aninsulative material such as a plastic. A magnet 110 has sides 110A and110B and ends 110C and 110D. End 110C can be a north pole and end 110Dcan be a south pole. A steel pole piece 112 may be mounted on magnetside 110B. Pole piece 112 improves the shape of and guides the fluxfield generated by magnet 110. Pole piece 112 may be omitted if desired.Magnet 110 and pole piece 112 are mounted in and retained by cavity 104.

Magnet 110 can be formed from molded ferrite or can be formed fromsamarium cobalt or neodymium iron boron. Magnet 110 has a changingpolarity along the length of the magnet.

The magnet 110 could also be a tapered magnet or could be a magnet thathas a variable field strength along its length. The magnet 110 may havea variable polarization or a variable magnetic domain alignment alongits length. Magnet 110 may also comprise more than one magnet and may beseveral magnets.

Magnet assembly 100 is mounted in sensor opening 72 and rests on rail46. Bearing rail 46 extends into mortise or slot 106 such that magnetcarrier 102 is supported by bearing rail 46. Magnet carrier 102 slideson bearing rail 46 as the housing 40 moves relative to the anchor plate60.

A spring carriage 120 is between anchor plate 60 and housing 40. Springcarriage 120 is mounted in cutout 63. Spring carriage 120 attenuatesmotions other than in the primary load direction between anchor plate 60and housing 40. In other words, spring carriage 120 prevents rattling.Spring carriage 120 has a unshaped body 122 that has legs 124, 126 and abottom portion 128. An opening 130 is located between legs 124 and 126.A spring tab 132 extends into opening 130.

Four spring fingers 140 are mounted to body 122. One spring finger islocated at each corner of body 122. Spring fingers 140 have an uppertang 142 and a lower tang 144. Spring fingers 140 extend from cutout 63onto the top and bottom surfaces of anchor plate 60. Upper tang 142 isin contact with surface 60A. Lower tang 144 is in contact with surface60B. Anchor plate 60 is squeezed between tangs 142 and 144.

Spring tab 132 fits into slot 97 and is able to press against spring 90.Spring tab 132 applies a reverse force to spring 90 and assists withovercoming geometrical tolerance issues due to variations in thedimensions of the components. Spring tab 132 also assists with alignmentof spring 90 with respect to housing 40.

A bar 146 extends over magnet carrier 102. Bar 146 retains magnetcarrier 102 in opening 72.

Seat belt tension sensor 20 can be attached to a vehicle floor or seator other member (not shown) by a fastener 200 such as a bolt, rivet orscrew. Fastener 200 goes through aperture 64 and is attached to avehicle structure or seat. The fastener shown is threaded; however,other types of fasteners would work such as a rivet.

When a tension is applied to seat belt 30, housing 40 moves relative tothe fixed anchor plate 60 resulting in the compression of spring 90. Ashousing 40 moves, hall effect device 82 is moved relative to magnetassembly 100 which is held fixed by fastener 200 through anchor plate60.

As the tension in the seat belt increases, housing 40 will move furtherin relation to anchor plate 60. This causes the hall effect device 82 tomove. At the same time bearing rail 46 slides within slot 106. The totaltravel distance can be about 1 to 3 millimeters. The hall effect deviceis located adjacent to magnet 110. A small air gap is located betweenhall effect device 82 and magnet 110. The hall effect device outputs anelectrical signal that is proportional to the flux density of theperpendicular magnetic field that passes through the device. Since, themagnets have a north and south pole, the strength of the magnetic fieldvaries as the polarity changes from one pole to the other along thelength of the magnet.

Therefore, the resulting electrical output signal of the hall effectdevices changes in proportion to the amount of tension in seat belt 30.This electrical signal is processed by electronic circuitry and providedto an external electrical circuit through terminals 56 to a conventionalair bag controller or occupant classification module (not shown). Theair bag controller can then use the seat belt tension information tocompute a more accurate profile of the seat occupant and use thatinformation to control deployment of the airbag.

The movement of housing 40 relative to the fixed anchor plate 60 islimited by the interaction of flange 43 with edges 65A and 65B. In aresting position with no tension placed on seat belt 30, spring 90applies a force between arm 68 and wall 96 which results in the flange43 moving into contact with edge 65B. After flange 43 touches edge 65B,housing 40 can no longer move toward end 61 of anchor plate 60. Thisposition is defined as a rest or no tension position.

As tension is applied to seat belt 30, housing 40 will move away fromend 61 of anchor plate 60 and spring 90 will start to be compressed.Housing 40 will move relative to anchor plate 60 and therefore halleffect device 82 will move relative to magnet 110.

As further tension is applied to seat belt 30, flange 43 will move intocontact with edge 65A. After flange 43 touches edge 65A, housing 40 canno longer move away from end 61 of anchor plate 60. This position isdefined as an overload position.

Any further tension applied to seat belt 30 after flange 43 engages edge65B will be transferred to anchor plate 60 and bolt 200. The transfer ofadditional tension prevents further compression of spring 90 andprotects magnet assembly 100 and hall effect device 82 from damage dueto the possible application of large tension forces. This can bereferred to as overload protection.

The use of seat belt tension sensor assembly 20 has many advantages.Seat belt tension sensor assembly 20 allows for the measurement of seatbelt tension in a compact package with a small number of components.Seat belt tension sensor assembly 20 has a small amount of motion whilestill being able to determine the amount of tension in a seat belt. Seatbelt tension sensor assembly 20 has an overload protection mechanismthat prevents excessive seat belt tension from damaging the sensingcomponents.

The seat belt tension sensor of the present invention has additionaladvantages. It allows accurate sensing of seat belt tension, withoutexcessive movement of the seat belt. The seat belt tension sensor allowsan airbag controller to make better decisions as to when and how todeploy and airbag based upon more accurate seat occupant information. Inthe case of a child's car seat being strapped into a car seat, the seatbelt tension sensor in conjunction with a seat weight sensor allows theairbag controller to properly compute that the seat occupant has a lowweight and to prevent deployment of the airbag.

While the housing with an integral connector was shown used incombination with a magnet and magnetic field sensor, any suitable typeof sensor can be used with the integral connector. For example a straingage sensor could be used in combination with an integral connector.Other sensors such as inductive, optical, capacitive or pressure couldalso be used with an integral connector.

While the invention has been taught with specific reference to theseembodiments, someone skilled in the art will recognize that changes canbe made in form and detail without departing from the spirit and thescope of the invention. The described embodiments are to be consideredin all respects only as illustrative and not restrictive. The scope ofthe invention is, therefore, indicated by the appended claims ratherthan by the foregoing description. All changes which come within themeaning and range of equivalency of the claims are to be embraced withintheir scope.

1. A seat belt tension sensor for attachment between a seat belt and avehicle structure comprising: a) a housing; b) an anchor plate at leastpartially mounted in the housing, the housing being movable relative tothe anchor plate; c) a magnet retained by the anchor plate; d) a sensormounted to the housing, the sensor generating an electrical signal inresponse to relative movement between the housing and the anchor plate,the electrical signal changing in proportion to the amount of tension onthe seat belt; and e) a spring mounted in association with the housingand the anchor plate, the spring biasing the housing from the anchorplate.
 2. The seat belt tension sensor according to claim 1, wherein theanchor plate is in a fixed position and the housing is moveable.
 3. Theseat belt tension sensor according to claim 1, wherein the magnet is ina fixed position and the sensor moves.
 4. The seat belt tension sensoraccording to claim 1, wherein the anchor plate has a cutout, the magnetbeing mounted in the cutout.
 5. The seat belt tension sensor accordingto claim 4, wherein the magnet is mounted in magnet carrier, the magnetcarrier located in the cutout.
 6. The seat belt tension sensor accordingto claim 5, wherein the magnet carrier has a slot and a cavity, themagnet mounted in the cavity.
 7. The seat belt tension sensor accordingto claim 6, wherein the housing has a rail, the rail being mounted inthe slot, the rail further sliding in the slot.
 8. The seat belt tensionsensor according to claim 4, wherein the anchor plate has a first andsecond aperture, the cutout located between the apertures.
 9. The seatbelt tension sensor according to claim 8, wherein the seat belt passesthrough the first aperture.
 10. The seat belt tension sensor accordingto claim 4, wherein a projection extends from the anchor plate into thecutout.
 11. The seat belt tension sensor according to claim 1, wherein aconnector is attached to the housing.
 12. The seat belt tension sensoraccording to claim 11, wherein at least one terminal is mounted in thehousing and extends between the sensor and the connector.
 13. The seatbelt tension sensor according to claim 8, wherein the housing has anapertured flange, the flange extending into the first aperture.
 14. Aseat belt tension sensor comprising: a) housing means; b) plate means atleast partially mounted in the housing means, the housing means beingmovable relative to the plate means; c) flux generating means forgenerating a flux field, the flux generating means connected with theplate means; d) sensor means for sensing the flux field, the sensormeans connected to the housing means, the sensor means generating anelectrical signal in response to relative movement between the housingmeans and the plate means, the electrical signal changing in proportionto the amount of tension on a seat belt; and e) biasing means mounted inassociation with housing means and the plate means for biasing thehousing means from the plate means.
 15. A seat belt tension sensor forattachment with a seat belt comprising: a) a housing; b) an anchor plateat least partially mounted in the housing, the housing being movablebetween a first and a second position. c) a magnet held by the anchorplate; d) a sensor mounted to the housing, the sensor juxtaposed to themagnet; and e) a spring mounted between the anchor plate and thehousing, the spring being compressed as the housing moves away from thefirst position.
 16. The seat belt tension sensor according to claim 15,wherein the anchor plate and the housing together limit the maximumtension.
 17. The seat belt tension sensor according to claim 15, whereinthe anchor plate has a cutout.
 18. The seat belt tension sensoraccording to claim 15, wherein the housing has a spring channel, thespring at least partially mounted in the spring channel.
 19. The seatbelt tension sensor according to claim 15, wherein the housing has anintegral connector.
 20. The seat belt tension sensor according to claim15, wherein the sensor slides by the magnet.
 21. The seat belt tensionsensor according to claim 15, wherein a spring carriage is mounted withthe anchor plate.
 22. The seat belt tension sensor according to claim15, wherein the magnet is mounted in a magnet carrier.
 23. The seat belttension sensor according to claim 22, wherein the magnet carrier slideson a rail.
 24. The seat belt tension sensor according to claim 23,wherein the magnet carrier has a slot, the slot being engaged with therail.